Phosphorus modification is a known method of improving the performance of zeolite catalysts for a variety of chemical processes including, for example, the conversion of methanol to hydrocarbons and the methylation of toluene to produce xylenes. For example, U.S. Pat. Nos. 4,590,321 and 4,665,251 disclose a process for producing aromatic hydrocarbons by contacting one or more non-aromatic compounds, such as propane, propylene, or methanol, with a catalyst containing a zeolite, such as ZSM-5. The zeolite is modified with phosphorus oxide by impregnating the zeolite with a source of phosphate ions, such as an aqueous solution of an ammonium phosphate, followed by calcination.
In addition, U.S. Pat. No. 7,662,737 discloses a process for producing a bound phosphorus-modified zeolite catalyst, in which a zeolite, such as ZSM-5, which may be in the NH4+ or the H+ form, is slurried with an aqueous solution of a phosphorus compound and then water is removed from the slurry to form a phosphorus-modified zeolite. The phosphorus-modified, pre-calcined zeolite is then mixed with an acid-treated inorganic oxide binder material and, after optional extrusion, the zeolite-binder mixture is heated at a temperature of about 400° C. or higher to form a bound zeolite catalyst. The catalyst is particularly intended for use in the alkylation of toluene with methanol to produce xylenes, but is also said to be useful in MTG processes.
Similar processes of producing phosphorus-modified toluene methylation catalysts are disclosed in U.S. Pat. Nos. 7,285,511, 7,304,194, 7,368,410, 7,399,727, and 7,507,685, and in U.S. Patent Application Publication Nos. 2008/0275280 and 2009/0036723.
U.S. Pat. No. 6,504,072 discloses selective production of para-xylene by the reaction of toluene with methanol over a severely steamed ZSM-5 catalyst combined with oxide modifier, preferably an oxide of phosphorus, to control reduction of the micropore volume of the material during the steaming step. Incorporation of phosphorus in the catalyst is conveniently accomplished by contacting the ZSM-5, either alone or in combination with a binder or matrix material, with a solution of an appropriate phosphorus compound, followed by drying and calcining to convert the phosphorus to an oxide form.
However, existing methods of phosphorus modification of zeolite catalysts tend to add significantly to the complexity and hence the cost of the catalyst production process. There is therefore significant interest in the development of a higher throughput and lower cost manufacturing procedure for making phosphorus-modified zeolite catalysts.
According to the present invention, a more efficient, lower cost process for producing a phosphorus modified zeolite catalyst is proposed in which, after separation of the as-synthesized zeolite crystals from the mother liquor, but before substantial drying, the crystals can be subjected to NH4+ ion-exchange and phosphorus treatment. The resultant NH4+-exchanged, phosphorus treated zeolite crystals can then be formed into the required shape and calcined, e.g., to remove the organic directing agent and convert the zeolite from the NH4+ form to H+ form.